In recent years, in the automotive field, in order to reduce the weight of a vehicle body of a motorcar while maintaining or improving collision safety of the motorcar and reduce carbon dioxide emissions so as to improve the environmental performance of the motorcar, there are many approaches for improving the rigidity of a frame member such as a crash box, etc., of the motorcar, by modifying a cross-section of the frame member. In order to increase the rigidity of the frame member, a distribution of sectional property (or strength balance) of the frame member in the longitudinal direction thereof is important. If the design of the frame member is inappropriate, in the case of a forward collision of the motorcar, a frame positioned at a rear side of the crash box may be deformed before the deformation of the crash box positioned at a front of a frame of the motorcar. Further, in a collision experiment of a motorcar, a boundary condition such as a loading direction is not constant, whereby a certain degree of error occurs. Therefore, it is necessary that an energy absorbing member, a major deformation mode of which is a crushing mode in the axial direction of the crash box, etc., be highly robust, wherein impact-absorbing performance of the member is not considerably changed due to a change in the boundary condition.
In this regard, the “strength balance” means a second moment of area calculated from a plurality of cross-sectional shapes perpendicular to the longitudinal direction of the frame and characteristics of material applied to the frame, and/or a distribution of maximum tolerable buckling load in the longitudinal direction. Further, the “impact-absorbing performance” means an amount of energy absorption per unit amount of crushing in the axial direction, and the “robustness” means invariance of the impact-absorbing performance with respect to the change in the dynamic boundary condition.
As prior art, patent literature 1 discloses an impact absorbing member having a groove concaved toward inside the member, wherein a cross-sectional shape in at least a part in the axial direction is a closed cross-section having a plurality of vertices.
Patent literature 2 describes an energy absorbing member constituted by an extruded member of aluminum alloy having a hollow rectangular cross-section, wherein a rectangular cross-sectional projecting part is arranged outside a wall surface part of the member.
Patent literature 3 discloses a front side frame of a motorcar having beads on a lateral side thereof, the beads extending in the axial direction and projecting inside or outside the lateral side.
Patent literature 4 describes an impact absorbing member having a generally C-shaped cross-section which opens outward in the vehicle width direction.
Further, patent literature 5 describes an impact absorbing member having a polygonal cross-section, wherein the length of one side of the polygonal cross-section, the lengths of two sides which sandwiches the one side, and a range of the angle constituted by the two sides are limited.